Question: Which of these numbers is prime? ${14,\ 19,\ 69,\ 87,\ 99}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 14 are 1, 2, 7, and 14. The factors of 19 are 1 and 19. The factors of 69 are 1, 3, 23, and 69. The factors of 87 are 1, 3, 29, and 87. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 19 is a prime number.